1. Field of the Invention
The present invention relates to a method for producing a low alloy hot rolled steel strip or sheet having high tensile strength, low yield ratio and excellent total elongation.
Conventionally, high tensile strength steels have been used mostly as structural thick gauge steel plates. However, in recent years, various industries, such as, the automobile, railway and industrial machinery industries, have been increasing their use of hot rolled high tensile strength steel sheet for the purpose of weight and cost reduction.
Furthermore, the high tensile strength low alloy steel strip or sheet (herein collectively referred to as sheet) which is used in these fields is very often rigorously pressformed. Therefore, various problems are encountered with conventional high tensile strength steels. For example, they cannot be rigorously worked due to their low elongation and the precision of the formed articles produced from these materials is often unsatisfactory because of their high yield strength which causes them to warp and spring back after their deformation.
Therefore, high tensile strength steel sheet having the properties of excellent total elongation and a low yield ratio has been suggested as a material which would solve the above-mentioned problems. As a result, much work has gone into the development of this type of steel sheet because of the great need for such a product by users of steel sheet. The standards required for such steel sheet are a tensile strength of not lower than 40 kg/mm.sup.2, a low yield ratio of not higher than 70%, and an elongation better than that of conventional high tensile strength steel sheet.
2. Description of the Prior Art
A typical technique for producing this kind of low yield, high tensile strength hot rolled steel sheet involves annealing hot rolled steel sheet at a temperature not lower than the A.sub.1 transformation temperature and rapidly cooling it, using various heat treatment equipment, such as, a continuous annealing line, in order to lower the yield ratio. However, this type process greatly increases the production cost due to the additional heat treatment step required.
Another technique for obtaining high tensile strength sheet having the above-described qualities in the hot rolled state is by producing a dual-phase steel sheet containing a bainite structure by adding elements, such as, Mo, Ni, etc., in order to improve the hardening. The addition of such elements inevitably increases the production costs so that it is disadvantageous to use this steel material as steel sheet for automobiles or for other purposes which require a low cost sheet.
An alternative technique for producing a dual phase strip or sheet having a high tensile strength in its hot rolled state is by coiling a sheet composed of C, Mn, and Si as the essential alloying elements, i.e., what is known in the art as plain, low carbon steel, at an extremely low temperature, for example, at a temperature not higher than 300.degree. C.
To accomplish this requires the solution of various quality problems concerned with coil configuration and cracking, problems of operational techniques, such as, obtaining an accurate coiling temperature, and problems with respect to the facilities, such as, obtaining equipment with the requisite coiling capacity.